An increasing number of light fixtures are utilizing light emitting diodes (LEDs) as light sources to increased efficiency and provide a longer operational lifetime over conventional incandescent light sources. While designers using incandescent light sources have had decades to work out problems, LEDs are relatively new and still present some issues that need to be resolved before gaining wide acceptance. One such issue involves the reaction of LEDs to heat. LEDs require a relatively low constant temperature in comparison to incandescent light sources or bulbs. A typical operating temperature of an incandescent filament is over 2,000 degrees Celsius. An LED may have a maximum operating temperature of approximately 150 degrees Celsius, and operation above this maximum can cause a decrease in the operational lifetime of the LED. The decrease in light output is caused at least in part by carrier recombination processes at higher temperatures and a decrease in the effective optical bandgap of the LED at these temperatures. A typical operating temperature of an LED is usually below about 100 degrees Celsius to preserve operational lifetime while maintaining acceptable light output.
Multiple LEDs are typically grouped together in each light fixture to provide the amount of light output necessary for lighting a room in a home or building. LEDs used in light fixtures are typically considerably higher in light output and power consumption than the typical colored indicator LED seen in many electronic devices. This increase in the LED density and power causes an increase in heat buildup in the fixture. In LEDs, an increase in temperature causes an increase in current which, consequently, causes a further increase in temperature. If left unchecked, the increased current caused by increased temperature can cause thermal runaway where the temperature increases to a point where the LED is damaged. Therefore, it is important to control the power supplied to the LEDs to ensure that the temperature of the LEDs does not exceed the maximum safe operating temperature. Controlling the power to the LED can generally be accomplished by controlling the current or controlling the voltage, although light output is directly related to current.
Incandescent and fluorescent lighting fixtures in buildings are usually supplied by a line or mains voltage, such as 115 Volts AC at 60 Hertz in the United States. Other single phase voltages are also used, such as 277 Volts AC, and in some instances other single and multiple phase voltages are used as well as other frequencies, such as in Britain where 220 Volts at 50 Hz is common. Power to these lighting fixtures is controlled by a wall mounted switch for an on or off operation, and a dimmer switch can be used to control brightness levels in addition to providing a simple on and off function.
LEDs in light fixtures operate on a much lower voltage than what is typically supplied to a building. LEDs require low voltage DC so supply power must be converted from higher voltage AC to DC constant current. Generally a single white LED will require a forward voltage of less than approximately 3.5 Volts. It is also important to control current to the LED since excessive current can destroy the LED and changes in current can lead to undesirable changes in light output.
Some conventional LED lighting systems use thermocouples or thermistors to measure temperatures of the LEDs. These devices are placed in a position near the LED and are connected to a temperature monitoring system using set of wires that are in addition to the wires powering the LED. These temperature detection devices cannot directly measure the actual temperature of the LED die itself since they necessarily have to be spaced apart from the LED die because of optics of the LEDs and the LED conductors. In addition, the extra set of wires between the thermistor and the monitoring system can be inconvenient, especially if the monitoring system is a significant distance from the thermistor. Because the thermistors do not directly measure the actual temperature of the LED die, these devices introduce some particular inaccuracies into the temperature measurement.